This invention relates to a method of determining the thermal conductances and impedances of thermally resistive bonding layers used in infrared photoconductors.
In use of an epoxy to bond HgCdTe crystal detectors onto an Irtran 2 or sapphire substrate which is mounted onto a heat sink by a varnish, it has been determined that the epoxy and varnish bonding layers are thermally resistant and restrict heat flow from the detector to the heat sink. Further, the thermal conductances of these layers vary considerally from detector to detector.
It has been determined that the characteristics of thermal recovery processes are found to be quite sensitive to the specific details of the detector construction. Thermal recovery takes place in two separate time periods. Initially the signal recovers partially to an intermediate level on a time scale of several milliseconds. The rest of the recovery occurs much more slowly, on the order of hundreds of milliseconds. It has been determined that the magnitude of the thermally induced signal, the relative importance of the two recovery processes and the exact shape of the thermal recovery curve vary with power density and irradiation time.
Heretofore techniques for measuring the thermal characteristics of detectors have been restricted to DC bias current effects. This technique measures the thermal conductance of the total assembly without any capability of obtaining the thermal conductance of each layer. Also, the method only provides information on the steady state power handling capability of the detector without any information on pulse response.